i suspect the fake kogoro as culprit beside he could zap from the landlady house to change the chanel and sound while investigating and he might started this as a part of the plan to murder that dude and maybe that fake kogoro resided there once
no problem jinpei was sent to tokyo cause 3 years after hagiwara was killed the bomber came back and the police from there was afraid jinpei could make something drastic to avenge his death partner
i remember reading his data but don't remember his name but he's the brother of someone its all i remember
ok so after some research
kenji Hagiwara
he was jimpei partner and died 4 year before jinpei's entered the tokyo police